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A buried aeolian lag deposit at an unconformity between the Murray and Stimson formations at Marias Pass,
Gale Crater, Mars
H. Newsom, K Edgett, D Fey, R. Wiens, J. Frydenvang, S Banham, S. Gupta, A. J. Williams, J. Grotzinger, N. Mangold, et al.
To cite this version:
H. Newsom, K Edgett, D Fey, R. Wiens, J. Frydenvang, et al.. A buried aeolian lag deposit at an unconformity between the Murray and Stimson formations at Marias Pass, Gale Crater, Mars. 49th Lunar and Planetary Science Conference (LPSC 2018), Mar 2018, The Woodlands, TX, United States.
�hal-02902931�
HAL Id: hal-02902931
https://hal.archives-ouvertes.fr/hal-02902931
Submitted on 20 Jul 2020
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
A buried aeolian lag deposit at an unconformity between the Murray and Stimson formations at Marias Pass,
Gale Crater, Mars
H. Newsom, K Edgett, D Fey, R. Wiens, J. Frydenvang, S Banham, S. Gupta, A Williams, J. Grotzinger, N. Mangold, et al.
To cite this version:
H. Newsom, K Edgett, D Fey, R. Wiens, J. Frydenvang, et al.. A buried aeolian lag deposit at an unconformity between the Murray and Stimson formations at Marias Pass, Gale Crater, Mars. Lunar and Planetary Science Conference, Mar 2018, Houston, TX, United States. �hal-02902931�
A BURIED AEOLIAN LAG DEPOSIT AT AN UNCONFORMITY BETWEEN THE MURRAY AND STIMSON FORMATIONS AT MARIAS PASS, GALE CRATER, MARS H.E. Newsom1, K.S. Edgett2, D.M. Fey2, R.C. Wiens3, J. Frydenvang4, S.G. Banham5, S. Gupta5, A.J. Williams6, J.P. Grotzinger7, N. Mangold8, J. Schieber9, F. Ri- vera-Hernandez10, I. Belgacem1. 1U. New Mexico, Albuquerque, NM 87131, USA (Newsom@unm.edu); 2Malin Sp. Sci.
Sys., San Diego, CA, 3Los Alamos Nat. Lab, NM; 5Imperial College, UK; 6Towson U., MD; 7Caltech/Jet Prop. Lab, CA;
8IRAP/CNRS, FR; 9Indiana U., IA; 10U.C. Davis, CA.
Fig. 1. The Missoula lens of the Stimson Formation forms a coherent layer containing light toned clasts above the contact with the eroded Murray Formation lacustrine beds and below the Stimson cross-bedded sandstones. Area labeled “2” (inset c.) was inves- tigated with ChemCam LIBS analyses and MAHLI close-ups. Area labeled “1” (inset b.) contains a similar resistant layer just above the contact with the Murray that is also interpreted to be Missoula lens based on Mastcam images that also show light toned clasts (Fig. 2). We interpret the Missoula lens to be a thin aeolian lag deposit, consisting of fragments of Murray sediments and Ca- sulfate veins with a matrix of Stimson formation material, based on ChemCam analyses.
Fig. 2.
Close up of a portion of Area 1 (blue box in Fig. 1),
showing the uncon-
formable contact (red dots)
between Murray Formation and the Missoula lens. Area 1 is similar to Area 2 in containing light-toned clasts (white arrows), and subtle horizontal layering at the lower edge of the Missoula lens.
Introduction: The Mars Science Laboratory rover Cu- riosity reached the Marias Pass area, on Sol 992 as part of an effort to reach the contact between the Murray For- mation mudstone and sandstone lake deposits and over- lying cross bedded aeolian Stimson formation. A dis- crete layer or lens of material above the contact is out- lined in the images (Figs. 1, 2). We conducted an inten- sive study with ChemCam, MAHLI and APXS obser- vations of a portion of this lens dubbed “Missoula/Ro- nan” (Area 2 in Fig. 1), where the contact between the
Missoula lens and the Murray Formation is fully ex- posed [1].
Geology of the outcrop: The Murray Fm was first stud- ied at the Pahrump Hills, located 2-3 meters stratigraph- ically below the rocks exposed at Marias Pass. At Marias Pass, the Murray also consists of finely lami- nated mudstones and sandstones, suggesting lacustrine deposition [2]. The units above the contact are more complicated, but an intense study of the images and chemical data has led to a greater confidence in the in- terpretation of the materials in this area [3]. The lower- most Stimson just above the contact consists of an ero- sionally resistant layer or lens, up to 5 cm thick, desig- nated the Missoula lens, confirmed to exist only in the areas mapped in Fig. 1. This lens contains small rounded mm size lustrous and polished lithic mineral grains (Fig. 3). The lower 2.5 cm of the Missoula lens, called the Clark facies, also contains discontinuous hor- izontal layering and a concentration of irregular-shaped light-toned clasts (Fig. 1b, Fig. 2, 3). The light-toned clasts are probably pieces of mudstone (fine-grained with pores, e.g. Fig. 4) and fragments of calcium sulfate veins derived from the underlying Murray Fm. How- ever, ChemCam analyses of the lens material shows that the matrix sandstone supporting the clasts has a Stimson 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
chemical signature, and the lens is properly part of the Stimson Formation. Further complicating the story is the evidence for diagenetic silica enrichment in the lower most Clark facies along with occurrences of Ca- sulfate cement (Figs. 3, 4) within the Missoula lens and along the contact.
Fig. 3. Portions of MAHLI Clark mosaic. The Clark facies of the Stimson forms the lowermost layer of Stimson above the contact with the Murray formation. Some of the light-toned areas indicated with blue arrows appear to be white coated Ca- sulfate cement sockets where clasts, possibly of Murray mud- stone have been partially eroded out. Areas marked by white arrows could be angular pieces of calcium sulfate veins or Murray mudstone. Red arrows mark rounded lithic clasts.
Fig. 4 Portion of the Lumpry MAHLI observation of the Clark facies area. Note the irregular shapes denoted by white arrows. These appear to be Ca-sulfate cement casts in some cases with remnants of Murray mudstone. One clast, 1.7 mm across in the center of the image is surrounded by a white rind (probably calcium sulfate), with widths of 1 to 2 pixels for a maximum thickness of approximately 55 microns. The pit in the central clast is ~12 pixels across for a width of ~ 330 mi- crons. Note the recessed eroded nature of the clasts denoted by arrows. The strikingly spherical particle on the left could be an impact melt droplet (e.g. [6]).
Discussion: The sequence of events leading to the for- mation of the deposits in Marias Pass is complicated and provides evidence of a sedimentary cycle operating in the Martian past [4]. The initial deposition of the lacus- trine of the Murray formation was followed by for- mation of abundant Ca-sulfate veins. Subsequent ero- sion of the Murray resulted in a surface now forming the
contact with the overlying Stimson. However, there is no evidence from ChemCam analyses of weathering or paleosol alteration of the Murray close to the contact [3]. Deposition of the Stimson Fm on the contact began with the Missoula Lens in the area shown in Fig. 1. The Missoula lens consists of a lag of light-toned clasts of mudstone and eroded fragments of calcium sulfate veins eroded from the Murray [3]. We have observed fractures in the Murray, which contain sand-size material of Stim- son composition (Seeley target [3]), that are consistent with the introduction of Stimson composition material onto the surface of the eroded Murray. Although the Missoula lens has a Murray component, the matrix has chemical signatures of the overlying Stimson Formation (e.g. K2O/Al2O3), and the lens represents the initial dep- osition of Stimson formation material.
The nature of the Missoula lens is best explained as a lag deposit of eroded Murray material in small swales and depressions that was buried by encroaching Stimson sands, much like the situation near the current Bagnold dunes. An analogous lag of eroded material is present on many areas of Murray Formation today (e.g. Fig. 5).
Aeolian processes can result in evenly spaced angular or rounded clasts, being buried by a layered fine-grained matrix. Only after burial of the lag deposit, did cross- bedded Stimson deposits begin to form.
Fig. 5. Possible modern analogue for the Missoula lens of the Stimson Formation. Note the evenly distributed rock frag- ments, including pieces of Ca-sulfate, surrounded by aeolian sands. Fine layering is also evident in the distribution of dif- ferent grain sizes. Portion of Mastcam 100 mm image mcam08201 from sol 1605, ~8 cm across.
Following Stimson deposition, diagenesis of the low- ermost Missoula lens involved emplacement of calcium sulfate veins along the contact and enrichment of SiO2
up to 75 wt% in the Clark facies just above the contact [3, 5]. Ca-sulfate is also present in veins along contact and locally intruded the Stimson matrix locally cement- ing the sandstone.
References: [1] Newsom et al., 2016, LPSC. [2] Rivera- Hernandez et al., 2018, LPSC, this meeting. [3] Newsom et al.
2018, in preparation. [4] Edgett et al. (2018) LPSC, this meet- ing. [5] Frydenvang et al., 2017, GRL 44, 4716-4724. [6]
Newsom et al. 2015, Icarus 249, 108-128.
49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083)
